Project Summary/Abstract Obesity affects more than one out of every three U.S. adults and even higher proportions of African Americans and Latinos. While behavioral weight management interventions tend to have limited scope and are associated with only temporary small weight loss, environmental interventions, such as the provision of infrastructure to support physical activity, have the potential to reach large portions of the population and foster lasting weight control. Active transportation infrastructure (ATI) consists of physical structures that support safe walking and bicycling and include sidewalks, bicycle lanes and paths, and intersections with painted pedestrian crosswalks or pedestrian signals. To date, little is known about the relationship between neighborhood ATI and individual body mass index (BMI). Using secondary data from multiple sources, the proposed observational study will determine whether (a) adults who live in a neighborhood with greater ATI have healthier BMI and (b) adults who live in a neighborhood with greater ATI lose more weight in a weight management program than those living in a neighborhood with less ATI. It will also examine whether these relationships differ by race/ethnicity. The sample will consist of an urban subsample (n=232,000 adults) of the Weight And Veterans Environments Study (WAVES) I, which is a nationwide, retrospective longitudinal study of over 3.2 million veterans receiving healthcare from the U.S. Department of Veterans Affairs between 2009 and 2015 (R01CA172726). Using OpenStreetMap data, innovative neighborhood ATI measures will be constructed, including availability and density of sidewalks and bicycle lanes or paths and pedestrian intersection safety. Multivariable regression will estimate associations between ATI and BMI. This study will provide important evidence about whether enhancing ATI through policies and environmental modifications is likely to help people achieve or maintain a healthier body weight. In addition to supporting the completion of the proposed study, this fellowship will support additional research training for the principal investigator including in statistical analysis (e.g., secondary, longitudinal, and spatial data analysis). These experiences and skills will provide a strong foundation for an independent research career focused on enhancing urban environments to improve population and individual health outcomes.